Fix VirtualService validation added in Istio 1.0

[scothis]

Expected Behavior

The channel controller should create a valid VirtualService

Actual Behavior

The current VirtualService does not pass validation added in Istio 1.0

Steps to Reproduce the Problem

1. create a bus
2. create a channel
3. check the eventing controller logs or channel status for errors

Additional Info

This behavior is currently blocking the eventing e2e tests since serving updated Istio without eventing CI voting on the change.

/assign @scothis
/cc @tcnghia